Hi,
I went a little further than the challenge expected us to, I didn’t get that we should compose the image and the cube 
So I tried to compose the image and my truncated icosahedron… 
you know ? The Archimedean solid ? The football ? This ?
Annnnnd 2 problems popped up,
First the lighting: In cycles the Hemi-type light doesn’t exist, and in my scene the lighting is pretty even 
, well I used 2 lamps, I will need to tweak it later.
The second is that my ball got black parts and with the mix node it was transparent in the composition and I didn’t like it, I tried a bunch of stuff to add an Alpha Channel to my images and I finally found the Difference Key node after a bit of research on the blender manual [Blender manual link].
I guess we will talk about those points in the future lectures, so I went a bit too far.
Well here is the first compositing:
It sure is a big ball
And here is the node setup:
The good old green background was used, and two renders, one with object, lights etc (Image 1) and the other with only the background (Image 2) that we substract.
First compositing done, need to make it more realistic now 
Cheers !